Rockeries
Got sandy soil? Got rocky soil? Then maybe a rockery is for you! While the old saying is to plant a one dollar plant in five dollar soil (make that at least $5 and $25 in today’s world), some plants grow best in poor but well-drained soil.

In fact, many rock garden plants will rot if planted in good water-retentive soil. These are generally small mounding or sprawling plants that evolved above the timberline or in Mediterranean climates. Their natural conditions make them ideally suited for rocky, gritty or sandy soil. This makes them great choices for creating a garden in soil that might otherwise be considered a problem.

A rockery or rock garden with poor soil is the ideal showcase for these plants. While such a garden does not need visible rocks or boulders, these are always a good aesthetic choice. The real secret to these gardens, though, is good drainage. This is where rocky and sandy soil fits in. In fact, most rock garden experts recommend a soil that is one-third good soil or compost, one third sand and one-third grit or rocks.

This is especially critical if your garden bed is flat, is at ground level or does not receive full day sun – all conditions that work against good drainage. If you do not already have sandy soil, make sure that the addition of sand does not create a concrete-like hardpan: create a small test patch and then check its drainage.

The sand must be salt-free and coarser sand (builder’s sand) is better than very fine sand (masonry). If adding sand is not an option for your soil, then raising the bed, creating a slope or incorporating a small amount of fine rough gravel might be the solution.

New rockery plants are becoming available in our area, supplementing some old favorites. A good strategy for designing a rockery is to first select one or more dwarf evergreen shrubs that will provide some structure and year-round interest. A number of dwarf conifers such as Rheingold arborvitae (Thuja occidentalis Rheingold) are now available, and can be selected on the basis of size and color.

If you do not want or cannot obtain one of these, then consider the more freely available lavenders (Lavandula), heaths (Erica) or heathers (Calluna). Plant these shrubs away from the front edge of your rockery because they will grow bigger than the other plants you select.

Then have fun shopping for spring and summer interest plants. Spring bulbs generally do well in rockeries, but limit yourself to dwarf varieties like Crocus and Tulipa spp. (species tulip) that will not grow very tall – particularly if you plant them close to the front of the rockery. Aubrieta, Lithadora, Arenaria, snow-in-the-summer (Cerastium tomentosum), and yellow varieties of Allysum/Aurinia are good choices for low plants that sprawl and provide summer flowers.

Succulents like Sedum and hen-and-chicks (Sempervivum) can be placed in particularly sandy areas. And finally, the Rock Stars line of plants features a number of small, mounding plants that are best placed near the front of the rockery where you can admire them. An old pre-Rock Stars favorite of mine is sea thrift (Armeria maritima), although it can grow quite large with time. Since the plants you choose might differ in the amount of summer water they need, try to group plants by water needs.

If you do not have the full sun needed by most of the plants mentioned above, consider choosing dwarf shade- and drought-tolerant plants from among the fern and violet families. Cyclamen and hellebores can provide late winter/early spring bloom.

Once you have created your rockery, you must water your plants well the first summer because they need to establish good root systems. Since you will be watering regularly that first summer, you will probably want to mulch for weed suppression. Small rocks (scree) and sand are traditional choices for rockery mulches, but bark mulch can also be used.

After the first year, rockeries are relatively low-maintenance.
Flowering plants generally need a light shearing after blooming, and some plants will need some water during long dry patches. After the first year, most plants will appreciate a very light fertilization around the time of their blooming. But these occasional jobs will leave you plenty of time to enjoy your garden and our wonderful summers.

Most general gardening books will contain a section on rock gardens, crevice planting or scree gardens if you’d like to do some research. For an up-front and personal encounter, I strongly advise a trip to the Lohbrunner Alpine Garden section of the UBC Botanical Gardens.

(Rhiannon Allen is secretary of the Point Roberts Garden Club and director of the 2010 Garden Tour, which will take place on July 18. For many years, she couldn’t wait until the rock garden at the Brooklyn Botanic Garden would break forth in its late winter display of hellebores and spring bulbs, and admits to still tracking it online.)

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